1. Field of the Invention
This invention relates to the field of three dimensional (3-D) stereoscopic imaging and display technologies in general and relates more particularly to stereoscopic photography, television, motion picture, printing and computer displays.
2. Description of Related Art
The binocular vision of humans perceives the real world as 3-D images. This fact results from a combination of the physiological and psychological properties of the human pair of eyes. The pair of eyes is the most important source of depth perception. Because of its spherical shape, the retina of a single eye collects only two-dimensional image information. Therefore, cues of the third dimension (depth) can never be collected by the retina of a single eye.
The inventions of printing (1450) and photography (1839) enabled man to cheaply mass-produce pictures for popular use. There has always been the awareness that these technologies lacked the third dimension (depth), and therefore, the desire to invent ways to faithfully capture and reproduce nature as 3-D images has persisted throughout the ages. Around the year 1600, Giovanni Battista della Porta produced the first artificial 3-D drawing. The history and evolution of 3-D imaging techniques are surveyed in T. Okoshi, Three-Dimensional Imaging Techniques, Academic Press, New York, 1976, and T. Okoshi, Three Dimensional Displays, Proc. IEEE, 68, 548 (1980). Accounts of the most recent activities in 3-D technologies were recently presented in the Conference on Three-Dimensional Visualization and Display Technologies, in Los Angeles, Calif., Jan. 18-20, 1989 and published in the Proceedings of the International Society for Optical Engineers, SPIE volume 1083, edited by Woodrow E. Robbins and Scott S. Fisher.
3-D imaging is classified into two major classes: Autostereoscopic Imaging, a technique which produces 3-D images that can be viewed directly without the aid of wearing special eye-glasses; and Binocular Stereoscopic Imaging, a technique that requires wearing special eye-glasses.
Autostereoscopic Imaging
This 3-D class is further broken into four subclasses:
I.1 Parallax Barrier PA1 I.2 Lenticular Sheet PA1 I.3 Holographic PA1 I.4 Multiplanar free viewing PA1 1. It does not provide a full color display; PA1 2. It has been shown to lead quickly to eye fatigue; PA1 3. The image display is dark; PA1 4. The filter eye glasses are dark and cannot be used to view the natural 3-D environment when the viewer turns away from the artificial 3-D scene; PA1 5. Special new TV production equipment and transmission hardware are required; and PA1 6. Vertical misalignment leads to eye discomfort, fatigue, and headaches. PA1 1. Ghosting due to afterglow of the phosphors (slow decay) results in cross-talk between the left and right images affecting adversely the 3-D image quality. PA1 2. The LC shutter is angle dependent, affecting the quality and color of the 3-D image when the viewer's head is moved, and when more than one viewer is present. PA1 3. This technology cannot be adapted to hardcopy technologies such as plotting, computer printing, offset printing, instant photography, and conventional consumer photography. PA1 4. The system is very complex and very expensive, needs multiplexing electronics, scan converters, LC shutters and LC controllers. PA1 5. The technique cannot be used for TV. If the shutter frequency is compatible with TV then the flicker is unacceptable. If the frequency is higher than 60 Hz then it is incompatible with all the existing TV infrastructure, production equipment transmission equipment, receivers and VCR's. PA1 6. Freeze frame of live 3-D scenes is not possible. PA1 7. The technique cannot be adapted to the conventional motion picture infrastructure without discarding the massive investment in existing facilities.
The operating principles of these techniques are reviewed in detail in Three-Dimensional Imaging Techniques. The Parallax Barrier technique is one of the earliest techniques and was experimented with in the first quarter of this century. However, because of its complexity and the dark images it produced, it fell out of favor and was abandoned. The Lenticular Sheet technique is still used today to produce 3-D color postcards. It requires multi-cameras or one camera with multiple lenses. The recording process is quite complex and the final product is obtained after four or more images with different perspectives of the object are aligned and mounted to a sheet of a clear plastic cylindrical micro-lens array. Vertical misalignment results in discomfort and headaches. The technique becomes expensive for large prints, and has significant technical problems. Because of the use micro-lenses, the image does not look the same to different viewers in different viewing positions. Also the depth information in the image can be distorted and can depart significantly from the original object. Finally, this technique cannot be used for TV, computer displays, or computer printers.
The Holographic technique described in Three-Dimensional Imaging Techniques and Three Dimensional Displays, and by L. F. Hodges et al. in Information Displays, 3, 9 (1987), has shown promise for still images but not for TV or movies. It requires very sensitive and expensive film, and is unable to produce large holograms because of sensitivity to vibration. Also, because of the requirement for coherent monochromatic sources for recording and reconstruction (viewing), it is difficult to produce full color holograms. Holography is very expensive and is not used for mass markets as in conventional photography. The Multiplanar technique described by M. C. King and D. H. Berry, Appl. Opt., 9, May 1980; and R. D. Williams and F. Garcia Jr., SID Digest, 19 ,91 (1988), is the only member of the Autostereoscopic class which has a commercial product. It uses a verifocal mirror 1 which is a reflective membrane mounted on a speaker 2 as shown in FIG. 1. The speaker 2 causes the membrane to vibrate and to vary the focal length of the mirror. When the image of a cathode ray tube (CRT) 3 is viewed through this mirror it appears as a 3-D image 4. The information of the third dimension (depth) is represented by the different focal lengths which image different z planes of the original object. While this method has been successful in realizing a 3-D system for some special applications, it has serious limitations: i) it is not general purpose, i.e., the method cannot be used for 3-D hard copy production such as photography and printing; ii) it cannot be used for mass viewing as, for example, in a motion picture theatre; iii) it cannot be used for 3-D TV without making the massive investments in existing TV production equipment, broadcast equipment, TV sets, VCR, and other video equipment obsolete; and iv) has poor depth resolution given by the ratio of frame frequency to the mirror vibrating frequency. Other limitations such as cost and bulkiness, will limit the utility of this verifocal method.
Binocular Stereoscopic Imaging
This 3-D class has had the most relative success in narrow fields. To record an image, one generally requires two cameras, as illustrated in FIG. 2, one for the left image 5 and the other for the right image 6. In order to simulate the human stereoscopic vision, the cameras are separated by the pupil distance of 6.5 cm, which is the average distance between the two eyes. There are two techniques for coding the left/right information: Color Coding, as described in Three-Dimensional Imaging Techniques; and by L. Lipton, Foundations of the Stereo Cinema, Van Nostrand Reinhold, New York, 1982; and Polarization Coding, as described in Hartmann and Hikspoors, Information Displays, 3, 15 (1987); L. F. Hodges and D. F. McAllister, Information Displays, 5, 18 (1987); P. Bos et al., SID Digest, 19, 450 (1988).
Color Coding: In this technique, two different color filters 7,8 are placed in front of the cameras; for instance, green 8 in front of the left camera 5 and red 7 in front of the right camera. When the images are displayed as in FIG. 3, the viewer wears eye glasses 9 having the corresponding color filters, green, for the left eye, and red for the right eye. Thus, the left eye sees only the left image 10 (green image) of the object 15 taken by the left camera, while the right eye sees only the right image 11 (red image) taken by the right camera. This technique has been used for decades to produce movies with 3-D sensation and it can be used for 3-D TV. For real time TV (transmission and reception of live scenes), one requires extra hardware to synchronize TV cameras and to electronically combine the red with the green information before transmission. The TV receiver does not require any modification. This technique has several limitations:
Polarization Coding: Electromagnetic plane waves have electric and magnetic fields which are transverse to the propagation direction, as shown in FIG. 4. There are two possible orientations for each of the electric and magnetic fields. These orientations are called polarization states. The E1 and H1 transverse fields represent one wave having polarization P1 while the E2 and H2 transverse fields represent another independent wave having polarization P2 which is perpendicular to P1 as shown in FIG. 4.
Light from the sun, fluorescent lamps and incandescent bulbs is unpolarized, and is represented by electromagnetic plane waves which are an incoherent mixture of wavelengths, polarizations, amplitudes, and phases. Half of this light energy (luminance, or brightness) is in one polarization state, P1, while the other half is in other polarization state, P2. These two states of polarization are linear. Using quarter wave retarders with P1 and P2 states produce two circular polarization states, one is clockwise and the other is counter-clockwise.
It is possible to turn unpolarized light into linearly polarized light by one of three well known means: 1) Nicol prisms; 2) Brewster Angle (condition of total internal reflection in dielectric materials) ; and 3) Polaroid film. These are called linear polarizers. The most inexpensive and widely used polarizers are the Polaroid films. They are made of polyvinyl alcohol (PVA) sheets stretched between 3 to 5 times their original length and treated with iodine/potassium iodide mixture to produce the dichroic effect. This effect is responsible for heavily attenuating (absorbing) the electric field components along the stretching direction while transmitting the perpendicular electric field components. Therefore, if P1 is along the stretching direction of the PVA sheets, it is not transmitted, where as only P2 is transmitted, producing polarized light. By simply rotating the PVA sheet 90 degrees, P1 state will now be transmitted and P2 will be absorbed.
The two polarization states of light, P1 and P2, can be used to encode the left image and separately the right image to produce 3-D sensation, see L. Lipton, Foundations of the Stereo Cinema, Van Nostrand Reinhold, New York, 1982 and V. Walwarth et al., SPIE Optics in Entertainment, Volume 462, 1984. The arrangement of FIG. 2 without the filters 7,8 is used to record the left and right images separately. To reconstruct and view the images, the arrangement in FIG. 3 is used. The P1 polarizer 13 is placed in front of the left image 10 and in front of the left eye of the viewers eye glasses 9, and P2 polarizer 12 in front of right image 11 and in front of the right eye. The half-silvered mirror 14 is used to combine the left and right images. The viewers left eye sees only the left image because its polarizer P1 blocks the right image state of polarization P2. Similarly, the right eye sees only the right image. The brain fuses both images to perceive 3-D sensation.
This polarization coding technique, unlike the color coding, can reproduce color and therefore is considered the most promising for widespread applications. However, the arrangements (parallel field technique) shown in FIGS. 2 and 3 require two image sources, i.e., two CRTs or two computer displays, two movie projectors, or two slide projectors in addition to the half-silvered mirror combiner which loses half the brightness. For TV, it requires major investment in new production and transmission hardware because two separate channel frequencies are required for the left and right images.
As early as the 1940s, attempts were made to improve the polarization encoding technique by eliminating the need for two image sources during image reconstruction and viewing. As disclosed in U.S. Pat. Nos. 2,301,254 to Carnahan and 2,317,875 to Athey, et al., left and right perspective images of a three-dimensional object are effectively combined into a single composite image referred to therein as an "interlined stereogram". As disclosed in these prior art references, the composite image is produced during an image recording process by (1) recording left and right perspective images, (2) spatially-filtering these perspective images using complimentary optical masks, and thereafter (3) optically combining the spatially-filtered perspective images to produce the composite image. In its final form, the composite image contains thin spatially alternating image strips taken from the left and right perspective images. Then during the image reconstruction and viewing process, the composite image is displayed from a single image source (e.g. film projector) through a linear polarizing filter or screen consisting of vertically-interleaved cross-polarized light polarizing strips, physically aligned with the left and right image strips in the displayed composite image. As the composite image is being displayed through the light polarizing filter, the viewer views the polarized composite image through a pair of linear polarizing spectacles. In theory, the polarization encoding technique taught in these prior art references should permit one to view three-dimensional objects stereoscopically. In practice, however, an array of unsolved problems, relating to stereoscopic cross-talk and polarizing filter design and manufacture, has continued to prevent anyone from using this polarization encoding technique to build a n image display system that permits stereoscopically viewing of three-dimensional imagery with high image quality and resolution.
Field Sequential Technique J. S. Lipscomb, Proc. SPIE, 1083, 28 (1989)!: The major motivation to pursue this technique is to have a 3-D system that uses only one display screen (CRT) instead of two as was required in the parallel field arrangement of FIG. 3. This is accomplished by means of sequentially recording the left and the right scenes (fields) and then sequentially displaying them in the same order with proper synchronization. The sequential imaging, recording and displaying, require two fundamental pieces of hardware, during recording and during display. They are the time multiplexing electronics and the optical shutters or switches. As described by P. Bos et al., SID Digest, 19, 450 (1988), these shutters can be either electromechanical or electro-optical shutters. Electro-optical shutters are the most preferable and are used as representatives of the field sequential 3-D display prior art illustrated in FIGS. 5 and 6.
In FIG. 5 an electro-optical shutter 16 made of liquid crystal (LC) or piezo-electric (PZLT) crystal is placed in front of the left camera 5 and an identical shutter 17 in front of the right camera 6. The shutter electronics box 18 sends signals to these shutters to open them and close them sequentially at a predetermined frequency, fc. The video output 19 of the cameras 5,6 are combined by means of the time multiplexing electronics 20 and then sent to the transmitters, the processor or the display system. The two cameras and the shutters are driven by proper sync signals which also control the multiplexer 20. At the receiving end, to display the 3-D image, one needs an electro-optical shutter/polarization switch 22 in front of the CRT display 21 and a shutter controller box 23 and scan electronics 24 to drive the CRT. The display shutter has to be driven at the same frequency fc of the recording shutters 16,17. The LC shutter 22 switches to transmit the left scene with polarization P1 and the right scene with polarization P2 (P1 and P2 are either linear or circular polarizations). The viewer wears passive eye-glasses 9 with the corresponding states of polarization so that the left eye will see only the left scene and the right eye will see only the right scene, thus achieving the 3-D sensation. In FIG. 6, the LC shutter 22, which is the same size as the CRT, consists of a linear polarizer 25, the LC switch 26, and a quarter wave retarder 27. The eye-glasses 9 have two quarter-wave retarders 28 and two linear polarizers 29.
Because a large area LC shutter is very expensive, U.S. Pat. No. 4,562,463 by L. Lipton describes alternatives to the method of FIG. 6. These are shown in FIGS. 7 and 8. Instead of one large LC shutter in front of the CRT, two small LC shutters 30 are fixed to the eye-glasses 9. These shutters are either driven directly by a cable 31 (the viewer is teetered to the display system) FIG. 7, or by means of an infrared transmitter/receiver arrangement 32 shown in FIG. 8, and described by Hartmann and Hikspoors, Information Displays, 3, 15 (1987). The latter has a small battery fixed to the glasses to drive the receiver electronics 33 and the LC shutters 30. All the components within the dashed box 34 are mounted on the eye glasses 9 worn by the viewer.
Of all the 3-D prior art, the field sequential 3-D technique is considered state-of-the-art and is being used in some applications in conjunction with computer workstations, achieving acceptable 3-D sensation for those applications (see J. S. Lipscomb, Proc. SPIE, 1083, 28 (1989)). However, this technique has fundamental limitations which will confine it to a very narrow application niche. These limitations are:
A hardcopy technique using polarization coding is described by Edwin Land in J. Opt. Soc. of America, 30, 230 (1940) and is called a vectograph. It is made of two sheets of stretched, unstained PVA material laminated together with their transmission axes perpendicular. If iodine based ink is used to draw an image on one side of this laminate, the area covered by ink will polarize light through it while all unpainted areas leave the light unpolarized. Drawing on the opposite side likewise produces an image in polarized light but with polarization oriented perpendicular to the first. For stereo images, the left perspective is recorded on the front side of the vectograph and the right perspective on the back side. The 3-D image is viewed by using polarized glasses.
The image recording process of vectograph is complex. It uses the method of dye transfer from matrix films. One matrix film is exposed with the image of the left perspective, developed and then filled with iodine/dye ink. The ink saturated matrix is then pressed onto the front side of the vectograph, transferring the ink to it. The image on the vectograph is in the form of spatial variations of the amplitude of the polarization vector. The process is repeated to create the image of the right perspective on the other side of the vectograph. For color vectograph, color separation methods are used. In this case the process is repeated eight times, four colors (cyan, magenta, yellow, black) for each perspective. These eight images must be carefully aligned, for any vertical misalignment leads to eye discomfort, fatigue and headaches. The alignment of images on both sides of the vectograph is a major problem.
Clearly, the vectograph cannot be used for TV, computer displays, or computer color printing. For this reason and the complexity and the cost of the vectograph process, its use is limited to special black and white 3-D hardcopy applications.